Understanding kPa

General Topics for configuring, operating and tuning the Megajolt. Also see the <a href="http://www.autosportlabs.net/MJLJ_V4_Operation_Guide">Operation Guide</a>

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speedmerchant
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Understanding kPa

Post by speedmerchant »

Hello,

Newbie here who is about to do some mapping and needs to understand the relationship of kPa to vacuum and pressure. With vacuum you have units in in/hg and pressure psi. Is there a simple way of understanding the conversion?

I look at the kPa numbers on maps and have no idea as if I am looking at if its under vacuum or psi.

Best,

Jeff

Gilesy998
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Post by Gilesy998 »


brentp
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Post by brentp »

Now now, as fun as LMGTFY is, it's still nice to have a discussion about it. :P

KPa a measurement of absolute pressure.

0 KPa is a perfect vacuum
100-103 KPa is sea level air pressure
more than that is boost (super/turbo charging)

So you can see, it's a neat and clean way to measure pressure- and a way to determine engine "load".
Brent Picasso
CEO and Founder, Autosport Labs
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speedmerchant
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Joined: Mon Apr 05, 2010 12:33 am
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Post by speedmerchant »

I think I am starting to wrap my head around it. Never really had to consider what the vacuum canister was doing but now I do. My car has vacuum advance and boost retard for the canister along with mechanical advance. The centrifugal advance numbers are listed in my repair manual, however not so for the canister.

Have you found away to measure the stock vacuum advance or boost retard in order to use those number for for use in the mapping? The vacuum advance is ported off the carbs and pressure retard is off the plenum.

Thanks for the input,

Jeff
1983 Esprit Turbo

Gilesy998
Posts: 144
Joined: Tue May 16, 2006 4:35 pm
Location: Liverpoool, UK

Post by Gilesy998 »

brentp wrote:Now now, as fun as LMGTFY is, it's still nice to have a discussion about it. :P
Sorry dude, it was the inner twelve-year-old's fault! :lol:

As a rule, for day to day conditions on a NA car, wide open throttle will show 101-102kPa near sea level, you'll not get a perfect vacuum on an engine (it just wouldn't run) and for a turbo'd car +1BAR or +14.5psi of boost would show 200kPa on your screen. Remember that all everythings see 1BAR in atmospheric pressure at sea level, and a BAR or boost is actually twice atmospheric.

speedmerchant
Posts: 20
Joined: Mon Apr 05, 2010 12:33 am
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Post by speedmerchant »

If I had a 12 year old I am sure I would be solving this qucker! :D What I need to learn is under normal driving conditions, stop, go, cruise what will be the manifold pressures I will be dealing with. The boost seems like the easy part.

Best,

Jeff

Gilesy998
Posts: 144
Joined: Tue May 16, 2006 4:35 pm
Location: Liverpoool, UK

Post by Gilesy998 »

That all depends on the conditions i.e. engine speed vs. engine load vs. throttle opening.
Cruising on a flat motorway for example, the engine's turning at a medium speed but is under only a little load and thus the throttle's only part open, so you'll see about 2/3 vacuum.
Compare that to climbing a very steep hill behind a slow lorry; the engine's turning slowly, but is under high load and the throttle is almost wide-open and there's very little vacuum.
Lastly, rolling down the same steep hill in a short gear to brake the car's descent will show the engine no load, but the throttle's fully closed and there's almost a perfect vacuum.

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